As is known, once gear wheels are cut, they undergo various thermal treatments, which alter the original geometrical and/or dimensional characteristics of the gear wheels themselves.
For this reason, following upon thermal treatment, gear wheels must necessarily again undergo machining operations of finishing using machine tools.
Finishing operations are, however, subordinate to restoring or redefinition of the axis of symmetry or of rotation of the gear wheels.
For restoring the axis of rotation it is today known to couple a first axial end portion of the gear wheel, normally the portion without internal grooves, to a rotating head with horizontal axis and to reduce manually by trial and error the eccentricity of a second, axial, end portion of the gear wheel opposite to the first end portion by progressively turning the rotating head.
Once the eccentricity has been reduced to a minimum, the first end portion is clamped on the rotating head, and a centring surface is formed on the second, axial, portion.
Once said centring surface has been obtained, the gear wheel is taken down from the rotating head, is turned upside down, the second portion is mounted on the rotating head, and the eccentricity of the first portion is reduced before the second portion is clamped on the rotating head, and a centring surface is also formed on the first portion.
At this point, the gear wheel is taken down from the rotating head and mounted between the tips of a control machine, on which, once again manually and by reference and measurement equipment, a check is made to is verify that the residual eccentricity is acceptable for the subsequent machining operations to which the gear wheel is subjected after it has been taken down from the control machine and mounted between the tips of a machine for carrying out machining.
Even though the technique of restoring the axis of symmetry described above is used universally, it requires, in the first place, the availability of qualified staff with high experience and considerable manual skills and, in any case, entails restoring times that are particularly long in so far as it is difficult to achieve manually a condition of eccentricity compatible with the subsequent machining to be carried out on the gear wheel, above all in the presence of gear wheels that present particular deformations and/or large dimensions.
In addition to this, restoring of the axis requires the use of a wide range of machines, of a plurality of measuring instruments and specific equipment, many of which must be chosen, each time, as a function of the type and characteristics of the gear wheel to be machined.
Moreover, the technique of restoring the axis does not enable increase beyond given limits of the dimensional and geometrical precision owing to the fact that, between machining of one hub and the other, the gear wheel is taken down from the rotating head, turned upside down, and again repositioned and remounted on the rotating head.
Reaching of high levels of dimensional and geometrical precision is then hindered also by the fact that the formation of the aforesaid centring surfaces is obtained with the gear wheel set in cantilever fashion and by rotating the gear wheel itself.
Machines for machining gear wheels are described, for example, in US 2007/199187 A1 and U.S. Pat. No. 5,109,634 A.